1. Field of the Invention
The present invention relates to a method of forming a zinc oxide thin film and a process for a photovoltaic device using it.
2. Related Background Art
Conventionally, a photovoltaic device composed of hydrogenated amorphous silicon, hydrogenated amorphous germanium silicon, hydrogenated amorphous carbide silicon, microcrystalline silicon or polycrystalline silicon and the like has utilized a reflecting layer on the back surface of the device in order to improve a current collection efficiency in long wavelength. For such a reflecting layer, it is preferable that a reflecting layer shows effective reflection characteristics at wavelengths, at which the absorption of long wavelengths is reduced near the edge of a band in the semiconductor material, that is, 800 nm to 1200 nm. It is a metal such as gold, silver, copper and aluminum that sufficiently satisfies this condition. Also, it has also been performed to provide an optically transparent texture layer within a predetermined wavelength range, which is known as optical confinement, and generally, the texture layer may be provided between the metal layer and a semiconductor active layer to improve a short-circuit current density (Jsc) by effectively utilizing reflected light. Further, in order to prevent the characteristics from being deteriorated due to a shunt path, between this metal layer and the semiconductor layer, there has been provided a layer made of light-transmissive, conductive material, that is, a transparent conductive layer. Exceedingly generally, these layers are deposited by a method such as vacuum evaporation and sputtering, and have been improved by 1 mA/cm2 or higher in terms of short-circuit current density (Jsc).
As their examples, in prior art 1: xe2x80x9c29p-MF-2 Optical confinement effect in a-SiGe solar cells on a stainless steel substratesxe2x80x9d (1990 autumn) the Extended Abstract of 51st Science Lecture Meeting of the Japan Society of Applied Physics, p747, and prior art 2: xe2x80x9cP-IA-15a-SiC/a-Si/a-SiGe Multi-Bandgap Stacked Solar Cells With Bandgap Profilingxe2x80x9d, Sannomiya et al., Technical Digest of the International PVSEC-5, Kyoto, Japan, p381, 1990, and the like, the reflection factor and texture structure of a reflecting layer made of silver atom have been studied. In these examples, they disclose that an effective texture is formed by depositing as the reflecting layer two layers of silver different in substrate temperature, whereby increased short-circuit current due to the optical confinement effect has been achieved by means of combination with the zinc oxide layer.
A transparent layer to be used as the optical confinement layer as described above is deposited by means of the vacuum evaporation method using resistance heating or electron beams, the sputtering method, the ion plating method, the CVD method or the like, and high wages for preparing target materials and the like, a high depreciation cost for the vacuum apparatus and low utilization efficiency of the materials increase the cost of the photovoltaic device using these techniques to an exceedingly high cost, and cause great obstacles to apply the solar cells for industrial use.
As a measure against them, as zinc oxide-producing technique using the liquid phase deposition method, in Japanese Patent Application Laid-Open No 10-140373, there has been explained a combination of a metal layer and a transparent conductive layer to be applied as a reflecting layer of the photovoltaic device (solar cell).
According to these methods, it is possible to dramatically reduce the cost for producing zinc oxide because there is no need for an expensive vacuum apparatus and a high-cost target. Also, since it is possible to deposit also on a large-area substrate, this holds great promise for such a large-area photovoltaic device as the solar cell. However, these methods of electrochemically depositing zinc oxide have the following problems.
(1) A deposited film having texture structure having more effective optical confinement effect requires further study.
(2) Abnormal growth having a shape such as sphere and dendrite is liable to occur, and causes the adhesion to be reduced. Further, when this zinc oxide film is used as a part of the photovoltaic device, the abnormal growth causes a shunt path of the photovoltaic device to be brought about.
(3) Variations are prone to occur in the size of the zinc oxide crystal grains, and the uniformity when made into a large area poses a problem.
The present invention has been achieved in light of the above-described problems, and is aimed to establish a new technique of zinc oxide electrodeposition method, which reduces the cost, and to stably provide an excellent mass-producible, high-performance and low-cost substrate with zinc oxide electrodeposited thereon, which has an excellent adhesion property between the substrate and zinc oxide, and to contribute to full-scale proliferation of solar-electric power generation by incorporating the substrate with zinc oxide electrodeposited thereon into a photovoltaic device.
In order to accomplish the above-described object, concerning technique for forming a zinc oxide film having texture shape which has high light confinement effect on a conductive substrate, and further technique for improving the adhesion and film uniformity, the present inventors have reached the present invention having the following structure as a result of effortful researches.
(1) In a method for forming a zinc oxide film on a conductive substrate, which comprises dipping a conductive substrate and a counter electrode in an aqueous solution containing at least nitric acid ion and zinc ion and supplying a current between the conductive substrate and the counter electrode, wherein the aqueous solution further contains polycarboxylic acid in which a carboxyl radical is bonded to a plurality of carbons having sp2 hybrid orbital, or its ester, and wherein a concentration of the polycarboxyllc acid is set to 0.5 xcexcmol/L to 500 xcexcmol/L. Thereby, it is possible to deposit, with good efficiency, zinc oxide having texture structure excellent in optical confinement effect. In this respect, the carboxylic acid in the present specification also includes ionized carboxylic acid.
(2) As the above-described aqueous solution, there is used an aqueous solution containing at least nitric acid ion, zinc ion and polycarboxylic acid in which a carboxylic radical is bonded to each of adjacent carbons having sp2 hybrid orbital, or its ester. Thereby, it is possible to deposit, with good efficiency, zinc oxide having texture structure excellent in more effectively optical confinement affect.
(3) As polycarboxylic acid in which the carboxyl radicals are bonded to a plurality of carbon having the above-described sp2 hybrid orbital, or its ester, phthalic acid or its ester is used. Thereby, it is possible to deposit, with good efficiency, zinc oxide having texture structure excellent in more effectively optical confinement effect.
(4) The above-described zinc ion concentration is set to 0.05 mol/L or more. Thereby, it is possible to deposit zinc oxide, with good efficiency, having texture structure excellent in more effectively optical confinement effect.
(5) As the above-described aqueous solution, an aqueous solution containing saccharose or dextrin is used. Thereby, it is possible to deposit a zinc oxide film having excellent adhesion and less abnormal growth for many hours with good efficiency.
(6) As the above-described conductive substrate, a conductive substrate having a zinc oxide film deposited thereon in advance is used. Thereby, it is possible to deposit a zinc oxide film having excellent adhesion and less abnormal growth with good efficiency. Also, the crystallizability of the zinc oxide film which is deposited in advance is controlled (electrodeposition on a zinc oxide film having high or enlarged crystallizability can make the crystal grain size larger, or in the opposition case, the crystal grain size is made smaller, and as a factor affecting the crystallizability, there are, for example, film formation temperature, film formation rate, film thickness and the like), whereby it is possible to deposit, with good efficiency, zinc oxide having texture structure excellent in more effectively optical confinement effect.
(7) In a process for producing a photovoltaic device comprising a zinc oxide film, the zinc oxide film is formed by the above-described method and a semiconductor layer is further formed thereon. Thereby, it is possible to provide a photovoltaic device with high quality (excellent in short-circuit current, conversion efficiency, and adhesion) and low power cost.
As described above, according to the present invention, it is possible to form a zinc oxide film having high optical confinement effect, which is useful to improve the solar cell characteristics, and high reliability at low cost with stability for industrial use, and this contributes to improvement in current collection efficiency and reliability of the photovoltaic device.